This invention relates to improved electro-luminescent display devices and to a method of manufacturing such devices.
Electroluminescent display devices are known for use in calculators and the like. These devices are made of thin film laminates which incorporate a light-emitting film of a material such as zinc sulfide doped with manganese or rare earth elements which emits light in response to an applied electric field. These display devices utilize a "flat panel" construction whereby variable displays on a large screen are made possible through the incorporation of a large number of light-emitting picture elements arranged in matrix form.
In making such a device, it has been recognized that the light-emitting efficiency of the light-emitting film decreases when the electric field is applied directly to the film. Insulation films have therefore been incorporated into the device separating the light-emitting film from the electrodes. Such a device is shown in FIG. 4.
In FIG. 4, an electroluminescent device 10 is formed from a substrate 1 on which an array of transparent electrodes 2 is formed. The substrate 1 is a transparent insulating material, such as glass. The electrodes 2 are formed from a transparent conductive material such as indium tin oxide and are formed as a plurality of substantially parallel stripes on the surface of the substrate.
Over the electrodes 2 are formed an insulating film 3 formed from inorganic insulators such as silicon nitride and having a thickness of several thousand Angstroms; a light-emitting film 4 having a thickness of several thousand Angstroms; a second insulating film 5 similar to insulating film 3 and a second electrode array 6. The second electrode array is arranged at right angles to the first electrode 2 and may be made of a metal such as aluminum.
The display voltage DV applied to the electroluminescent device is applied across the transparent electrode array 2 and the second electrode array 6 such that the polarities normally switch between positive and negative within each frame cycle on the display as shown in FIG. 4. Under these conditions, the light-emitting film emits light DL through the substrate 1 at the crossing points of the two electrode arrays.
This electroluminescent device suffers from a serious drawback, however, because the display voltage required to drive the device is so high that the driving circuit tends to be large and thus of greater cost. In particular, in the case of the device as shown in FIG. 4, a display voltage of 200 V or greater is required for a display with a practical level of luminescence. As a result, a breakdown voltage of about 300 V is required for the integrated circuit device in order to drive the display. This increases the size of the chip and results in unavoidably higher costs.
The simplest way to reduce the display voltage in the electroluminescent device is to reduce the entire thickness of the laminated structure. However, even if the thickness of the light-emitting film 4 is reduced to the minimum required to obtain reasonable luminance (4000 to 5000 .ANG.) and the insulating films 3 and 5 are made at a thickness of 3000 .ANG. which raises their internal electric field intensity to 10.sup.5 V/cm or higher, it is still difficult to keep the display voltage below 200 V. Thinner insulating films introduces increased risk of insulation failure during operation and thus is not really acceptable. Similarly, the omission of one of the insulating films does not solve the problem because it becomes necessary to increase the thickness of the remaining insulating film.
It is an object of the present invention to provide electroluminescent devices and a method for their manufacture which have a reduced display voltage requirement without loss of reliability.